Power converters for consumer applications typically operate at partial load under standby conditions for a relatively large part of their lifetime. Whilst functioning in this mode, it is desirable to draw power from the mains supply at as low level as is conveniently possible. Therefore it is desirable to use a power converter design for the power supply which operates with a high efficiency not only under full load conditions, but also for partial load, and particularly for low power standby mode.
Power supplies operating under nominal “no load” conditions of operation need at least to convert a small amount of power in order to supply their own circuitry such as IC, resistive components and optocouplers. No load input power levels below, for example, 300 mWatt and input power levels below 1 Watt at 500 mWatts output are becoming increasingly common as standard requirements.
For powers larger than approximately 100 Watts at full load resonant LLC topology is of interest and commonly adopted due to its high efficiency, small volumes and high power density. However, one of the main disadvantages for resonant LLC topology is its relatively low efficiency under low load conditions (when operated in the most common operation mode, that is, using a 50% duty cycle). Losses in this mode of operation may be a multiple of the required standby power.
One known method of controlling an LLC topology, under low load conditions, is the so-called “burst-mode” method. In this method, the power converter is used under nominal operating conditions for a certain, relatively short, period, and then completely switched off, for a longer period. This method is disadvantageously noisy (in the audio range), and requires a large smoothing capacitance on the output side. Also the efficiency is limited, because a fixed frequency is applied above the normal operating frequency, necessary due to component tolerances, giving a relatively low power level during the burst.
A method of improving the efficiency of LLC topology power converters has been proposed in patent application publication WO2005/112238A2. This publication discloses a method wherein the timing of the two control switches is such that the high side switch (HSS) conducts for a short interval during which both the primary current increases to a certain level and magnetising energy is built up in the transformer. It is during this interval that most of the output current is delivered. At the end of this interval the HSS is turned off, and the low side switch (LSS) is turned on shortly after this moment (the duration of the gap being such as to facilitate soft switch-on of the LSS, as is well known by those skilled in the art). The output current rapidly decreases to zero. The magnetising current starts to resonate in an LCC resonant circuit which is defined by the resonant capacitor, leakage inductance and the magnetising inductance in series. At a moment corresponding to the Nth negative maximum of the magnetising current, the LSS is turned off. The integer N can typically have a value from 0 to several hundred. At that moment the half bridge node between the HSS and LSS is charged by the magnetising current and provides a soft switch-on for the HSS, ready for the next HSS conduction interval.
Although this method makes it possible to significantly increase the efficiency of the power converter, it has a disadvantage in that the LSS may be turned off only at moments in time corresponding to the Nth negative maximum of the magnetising current, where N is an integer. This has the result that the switching cycle period of the power converter is quantised, and can only take certain discrete values. This has implications for the possible values of the output power, as well as for the switching period. Further, this may result in an increased risk of audio noise and may give extra ripple in the output voltage. In particular a resonance of the magnetising current combined with a repetition frequency lying in the audio band is likely to increase the audio noise of the power converter. Furthermore, a resonating magnetizing current can also give rise to losses in the core, as well as conduction losses.